We examined the impact of methadone-maintenance treatment (MMT) on risk behaviors for transmission of bloodborne diseases in polydrug users who had tested positive or negative for hepatitis C (HCV). At intake, HCV-positive participants (n = 362) engaged in more HIV-risk behaviors (as measured by the HIV Risk-Taking Behaviour Scale; HRBS) than HCV-negative participants (n = 297) (p<.001). This difference was specific to injection-related behaviors and decreased significantly within the first few weeks of MMT (p<.0001). Where needles continued to be used, HCV-positive participants became more likely over time to engage in safer injecting practices. Furthermore, HCV-positive participants became more likely to use condoms than HCV-negative participants. These findings demonstrate that both drug- and sex-related risk behaviors decrease during MMT, and emphasize the benefits of methadone programs for public health and HIV/HCV prevention. In ongoing studies, we are investigating whether HIV risk behaviors are reduced by contingency-management interventions that reinforce cocaine and heroin abstinence, and we are piloting the use of handheld electronic diaries to measure HIV-risk behaviors in near-real time.